High tensile strength carbon nanotube film and process for making the same

ABSTRACT

A conductive carbon nanotube film having high tensile strength and initial tensile modulus comprises primarily oxidized small-diameter carbon nanotubes wherein the diameter of the small-diameter carbon nanotubes are at most about 3 nm. A method for making the film comprises refluxing an aqueous mixture comprising carbon nanotubes and an oxidizing agent to form a refluxed nanotube dispersion; forming a carbon nanotube film from the refluxed carbon nanotube dispersion; optionally removing nitric acid or other oxidizing agent from the carbon nanotube film; drying the carbon nanotube film; and heat-treating the carbon nanotube film to form a heat-treated carbon nanotube film. The method can also comprise sonicating the nanotubes prior to or after refluxing. A heat-treated small-diameter carbon nanotube film can have a tensile strength of over 70 MPa and an initial tensile modulus of about 5 GPa.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from provisional United Patent application Ser. No. 60/523,806, filed Nov. 19, 2003, which application is incorporated herein by reference.

This invention was made in part with United States Government support under Grant No. F49620-03-1-0124 awarded by the Air Force Office of Scientific Research and under Grant No. N00014-01-1-0657 awarded by the Office of Naval Research. Government may have certain rights in the invention.

FIELD OF THE INVENTION

This invention relates generally to carbon nanotubes, and more particularly to a high tensile strength film comprising carbon nanotubes.

BACKGROUND OF THE INVENTION

Small-diameter carbon nanotubes having diameters between about 0.5 and about 3 nanometers, commonly known as “buckytubes,” have been the subject of intense research since their discovery due to their unique properties, including high strength, stiffness, thermal and electrical conductivity. The walls of small-diameter carbon nanotubes are fullerenes consisting essentially of sp²-hybridized carbon atoms typically arranged in hexagons and pentagons. Some small-diameter carbon nanotubes have only one wall, and others have more than one. Large-diameter multi-wall carbon nanotubes (MWNT), having diameters in excess of about 4 nanometers, are multiple nested carbon cylinders. Because large-diameter multi-wall carbon nanotubes have substantially greater density of defects in their side-walls, they are, consequently, mechanically less strong and electrically less conductive than small-diameter carbon nanotubes. Additionally, compared to the large-diameter multi-wall carbon nanotubes, small-diameter carbon nanotubes have considerably higher available surface area per gram of carbon.

The exceptional mechanical properties of carbon nanotubes make them useful in composites requiring high tensile strength and modulus, such as in structural reinforcement for sports equipment, buildings, vehicles, ship hulls, aircraft, and artillery vehicle and personal body armor. Besides incorporating nanotubes dispersed in a matrix material, nanotubes in other forms, such as fibers and films, are useful in the construction of laminates for composite components of aircraft, automobiles and other structures. The fabrication of high strength films comprising small-diameter nanotubes for these and other applications remains a major challenge.

SUMMARY OF THE INVENTION

In one embodiment, the present invention relates to a conductive film comprising carbon nanotubes, wherein the film has high tensile strength and a high initial tensile modulus, and method for making the same.

In another embodiment, a method for making a conductive carbon nanotube film comprising refluxing an aqueous mixture comprising carbon nanotubes and an oxidizing agent to form a refluxed nanotube dispersion; forming a carbon nanotube film from the refluxed nanotube dispersion; drying the carbon nanotube film; and heat-treating the carbon nanotube film to form a heat-treated carbon nanotube film. In one embodiment, the method can further comprise removing the oxidizing agent from the carbon nanotube film. In another embodiment, the method can further comprise sonicating the nanotubes prior to or after refluxing. In yet another embodiment, the method can further comprise sonicating the nanotubes prior to adding the oxidizing agent. In another embodiment, the carbon nanotubes comprise single-wall small-diameter carbon nanotubes. In another embodiment, the carbon nanotubes comprise small-diameter carbon nanotubes wherein the small-diameter carbon nanotubes have a diameter of at most about 3 nm and can have one or more walls. In another embodiment, the forming of the carbon nanotube film is done by filtering.

In another embodiment, a film consists essentially of small-diameter carbon nanotubes, wherein the small-diameter carbon nanotubes are crosslinked.

In another embodiment, a heat-treated carbon nanotube film consists essentially of small-diameter carbon nanotubes and has a tensile strength of at least about 70 MPa (megapascals) and an initial tensile modulus of up to about 5 GPa (gigaPascals).

In yet another embodiment, a carbon nanotube film comprises primarily small-diameter carbon nanotubes, i.e. greater than about 50 wt % small-diameter carbon nanotubes, wherein the nanotubes have been oxidized, and wherein the film has a thickness in the range of about 0.1 micron and about 10,000 microns and has a tensile strength of at least about 15 MPa. In other embodiments, a film comprises primarily small-diameter carbon nanotubes and has a tensile strength of at least about at 25 MPa, at least about 50 MPa or at least about 70 MPa.

In another embodiment, a carbon nanotube film comprises small-diameter carbon nanotubes and has a tensile strength of about 74 MPa, i.e., a tensile strength that is more than seven times greater than that of a comparable film prepared without nitric acid treatment of the nanotubes.

In yet another embodiment, a carbon nanotube film comprises small-diameter carbon nanotubes and has an initial tensile modulus of up to about 5 GPa, an initial tensile modulus which is over six times greater than that of a comparable film prepared without nitric acid treatment of the nanotubes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the dynamic mechanical behavior of a film comprising small-diameter carbon nanotubes oxidized with 6 Molar (M) nitric acid.

FIG. 2A shows a Scanning Electron Micrograph (SEM) of a heat-treated film comprising small-diameter carbon nanotubes processed with 3M nitric acid.

FIG. 2B shows a SEM of a heat-treated film comprising small-diameter carbon nanotubes processed in 6M nitric acid.

FIG. 2C shows a SEM of a heat-treated film comprising small-diameter carbon nanotubes processed in 10M nitric acid.

FIG. 2D shows a SEM of a heat-treated film comprising small-diameter carbon nanotubes processed in 3M nitric acid, wherein the film was further subjected to a second heat-treatment at 900° C. in nitrogen.

FIG. 2E shows a SEM of a heat-treated film comprising small-diameter carbon nanotubes processed in 6M nitric acid, wherein the film was further subjected to a second heat-treatment at 900° C. in nitrogen.

FIG. 2F shows a SEM of a heat-treated film comprising small-diameter carbon nanotubes processed in 10M nitric acid, wherein the film was further subjected to a second heat-treatment at 900° C. in nitrogen.

FIGS. 3A-D show Raman spectra of the Radial Breathing Mode (RBM) peaks of films prepared by various embodiments of the present invention.

FIG. 3A shows the RBM peaks of a Raman spectrum for a heat-treated control film of small-diameter carbon nanotubes, wherein the film was further subjected to a second heat-treatment at 900° C. in nitrogen.

FIG. 3B shows the RBM peaks of a Raman spectrum for a heat-treated film comprising small-diameter carbon nanotubes which were oxidized in 3M nitric acid, wherein the film was further subjected to a second heat-treatment at 900° C. in nitrogen.

FIG. 3C shows the RBM peaks of a Raman spectrum for a heat-treated film comprising small-diameter carbon nanotubes which were oxidized in 6M nitric acid, wherein the film was further subjected to a second heat-treatment at 900° C. in nitrogen.

FIG. 3D shows the RBM peaks of a Raman spectrum for a heat-treated film comprising small-diameter carbon nanotubes which were oxidized in 10M nitric acid, wherein the film was further subjected to a second heat-treatment at 900° C. in nitrogen.

FIG. 4A shows a plot of the relative fraction of 1.1 g-nm diameter carbon nanotubes as a function of nitric acid concentration.

FIG. 4B shows a plot of the relative fraction of 1.11-nm diameter carbon nanotubes as a function of nitric acid concentration.

FIG. 4C shows a plot of the relative fraction of 1.07-nm diameter carbon nanotubes as a function of nitric acid concentration.

FIG. 4D shows a plot of the relative fraction of 1.02-nm diameter carbon nanotubes as a function of nitric acid concentration.

FIG. 4E shows a plot of the relative fraction of 0.89-nm diameter carbon nanotubes as a function of nitric acid concentration.

FIG. 4F shows a plot of the relative fraction of 0.88-nm diameter carbon nanotubes as a function of nitric acid concentration.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In this application, the terms “dispersion” and “suspension” are intended to have the same meaning and will be used interchangeably. A dispersion or suspension of carbon nanotubes is intended to be generally homogeneous, but may be temporary and unstable over time. For example, the carbon nanotubes may be dispersed in water during active sonication or mixing, but settle out in time afterwards. The carbon nanotubes can be present in the dispersion as individual nanotubes or nanotube bundles or ropes.

In one embodiment of the present invention, a high tensile strength and high tensile modulus conductive film consists essentially of small-diameter carbon nanotubes oxidized with nitric acid or other oxidizing agent.

The carbon nanotube material used to make the carbon nanotube film can comprise small-diameter single-wall carbon nanotubes, small-diameter multi-wall carbon nanotubes or a combination thereof. The nanotubes can be made by any known process for making carbon nanotubes, however, the composition of the nanotube material can influence the conductivity, strength and modulus of the nanotube film. For example, a nanotube material having more small-diameter carbon nanotubes with respect to large-diameter multi-wall carbon nanotubes and amorphous carbon, would be expected to be more conductive and stronger than a nanotube material with a lower concentration of small-diameter carbon nanotubes in a mixture of large-diameter multi-wall carbon nanotubes and amorphous carbon. In one embodiment of the invention, the carbon nanotube material comprises primarily single-wall carbon nanotubes (i.e. greater than 50 wt % of the carbon-containing material). In another embodiment, the carbon nanotube material comprises primarily small-diameter carbon nanotubes, in which the diameter of the small-diameter multi-wall carbon nanotubes is at most about 3 nm. Preferably, the carbon nanotube material comprises about 50 wt % to about 100 wt % single-wall carbon nanotubes or about 50 wt % to about 100 wt % small-diameter carbon nanotubes, in which the diameter of the small-diameter single-wall and/or multi-wall carbon nanotubes is at most about 3 nm, and in some embodiments between about 0.5 and about 3 nanometers.

The present invention involves film made with both small-diameter single-wall and/or multi-wall carbon nanotubes. Because small-diameter carbon nanotubes have a strong affinity for each other and are held strongly together by van der Waals forces, dispersing small-diameter carbon nanotubes is much more difficult than for large-diameter multi-wall carbon nanotubes. Therefore, procedures that are effective for dispersing and processing small-diameter carbon nanotubes are generally effective for multi-wall carbon nanotubes, however, the reverse is generally not the case, i.e., procedures that are suitable for multi-wall carbon nanotubes are generally not effective for small-diameter carbon nanotubes.

The carbon nanotubes can be made by any known means. The carbon nanotubes can be used as synthesized or after purification. Purification of the nanotube material can be done to remove amorphous carbon, metallic impurities and non-nanotube carbon. For certain applications, purification may be preferred and can be done by any known means. Suitable procedures for purification of carbon nanotubes are related in International Patent Publications “Process for Purifying Single-Wall Carbon Nanotubes and Compositions Thereof,” WO 02/064,869 published Aug. 8, 2002, and “Gas Phase Process for Purifying Single-Wall Carbon Nanotubes and Compositions Thereof,” WO 02/064,868 published Aug. 8, 2002, and included herein in their entirety by reference.

In one embodiment, a method for making a conductive film comprising carbon nanotubes wherein the film has high tensile strength and high initial tensile modulus comprises refluxing an aqueous mixture comprising carbon nanotubes and an oxidizing agent to form a refluxed nanotube dispersion; forming a carbon nanotube film from the refluxed carbon nanotube dispersion; drying the carbon nanotube film; and heat-treating the carbon nanotube film to form a heat-treated carbon nanotube film. The heat treatment can be carried out in an oxidizing environment, which may comprise liquid oxidizing agents, gaseous oxidizing agents or a combination thereof. The method can further comprise sonicating the nanotubes prior to or after refluxing. The method can further comprise sonicating the carbon nanotube suspension.

The oxidizing agent can comprise any compound that can oxidize carbon nanotubes. Oxidation can occur anywhere on the carbon nanotube, but, typically, occurs at the end caps and/or at defect sites of the nanotubes. Oxidizing agents that can oxidize carbon nanotubes, include, but are not limited to, nitric acid, ozone, potassium persulfate (K₂S₂O₈), sulfuric acid, sulfuric acid with hydrogen peroxide, oxygen, steam, carbon dioxide, halogens, halogen-containing compounds, sulfuric acid with nitric acid, and combinations thereof. For conciseness, nitric acid will be used as an exemplary oxidizing agent, even though other oxidizing agents could be used.

In another embodiment, the method comprises sonicating the carbon nanotubes in water prior to adding the nitric acid. Other mixing means can be used in addition to, or in lieu of, sonication to disperse the nanotubes in water. Other suitable mixing means include, but are not limited to, magnetic stirring, homogenization, grinding, high-shear mixing, application of high pressures between about 10 and about 100,000 pounds per square inch, and combinations thereof. Heat can also be applied before or during or after the dispersion process.

After sonication and/or suitable agitation of the nanotubes in water, nitric acid is added to the carbon nanotube/water mixture. Among other parameters, the concentration of the nitric acid can affect the tensile strength of the film. Generally, higher concentrations of nitric acid result in higher tensile strength carbon nanotube films. The concentration of nitric acid in the aqueous mixture is typically in the range of about 3 Molar and about 10 Molar. The nitric acid concentration can be in the range of about 3 Molar and about 6 Molar or in the range of about 6 is Molar and about 10 Molar. The concentration of nitric acid and other parameters can be adjusted to achieve the desired oxidation of the nanotubes and tensile strength of the film.

After the desired amount of nitric acid is added to the carbon nanotube-water mixture to obtain the desired nitric acid concentration, the acidified mixture can be sonicated, or otherwise agitated, again. As above, other suitable mixing means can be used with, or in lieu of, sonication. Agitation can be done from hours to days, such as from 12 hours to 3-4 days. A suitable amount of agitation is that time needed to effectively disperse the nanotubes.

After suitable sonication and/or agitation, the aqueous nitric acid-carbon nanotube mixture is refluxed to further disperse and oxidize the nanotubes. The reflux temperature is approximately the boiling point of the aqueous nitric acid mixture which can be generally in the range of about 100° C. and about 130° C. The refluxing time is dependent on the amount of nanotube oxidation desired. Higher concentrations of nitric acid will generally require less refluxing time. The refluxing time is the time to effectively oxidize the nanotubes to the amount desired. Typically, refluxing time can range from about 30 minutes to about 5 hours, more typically about 30 minutes to about 2 hours.

After refluxing, the refluxed aqueous nitric acid-carbon nanotube mixture can be sonicated, or otherwise suitably agitated, again. After sonication or suitable agitation, if done, a carbon nanotube film is formed. The nanotube film can be made any known means. In one embodiment, the nanotube film is formed by filtering the nanotubes from the mixture, such that the nanotubes are retained on the filter, and, thereby forming a film. Filtering the dispersion can be done through any suitable membrane and/or filter, with a pore size small enough that the nanotubes collect on a filter or membrane. “Filter”, “membrane” and “membrane filter” will be used interchangeably in this application. Examples of suitable filter material are polytetrafluoroethylene and Whatman filter paper. An example of a suitable filter pore size is about 1 micron. Whatman filter paper #1 is an example of a suitable filter. A suitable filter is any filter that takes out most of the nanotubes from the dispersion. Other means of forming a nanotube film from the nanotube dispersion can be used. For example, the nanotube dispersion could be poured on an absorbent or non-absorbent surface, and the solvent could be removed by any convenient means, such as, but not limited to, evaporation, heating, application of a vacuum, or combinations thereof.

At this point, the nanotubes can be peeled from the filter or other surface and dried. However, the nanotubes can also be washed to remove nitric acid from the nanotubes before drying. Repeated washings can be done with water or any suitable solvent, such as acetone, alcohols, such as methanol and ethanol, and combinations thereof. Preferably, the washing fluid is a polar compound with the ability to solubilize nitric acid and remove it from the nanotubes. The washing, if done, is conveniently performed when the nanotubes are on a filter, such that after washing the nanotubes, the nanotube film can be peeled from the filter and dried.

Drying the carbon nanotube film to remove moisture and any solvent from the nanotubes can be done by any known drying means, such as with heat, vacuum, ambient solvent evaporation, or combinations thereof. The drying atmosphere can be a vacuum or under nitrogen or inert gas, such as argon. The film can also be air dried. The drying is done at a temperature in the range of about room temperature, i.e., about 15-20° C., up to about 200° C. Typically, the drying temperature can be in the range of room temperature to about 100° C. The drying time and temperature are dependent on various parameters, including, but not limited to, the particular solvent used and the amount of water or solvent to be removed. The drying time is also dependent on temperature, ambient pressure and the thickness of the small-diameter carbon nanotube film. The amount of drying time is that amount of time required to remove most of the water or solvent.

After drying, the film is heat-treated to form a heat-treated carbon nanotube film. Heat-treating is generally done at higher temperatures than those for drying the film. Typical heat-treating temperatures are in the range of about 200° C. and about 1000° C. Heat-treating can be done in a vacuum or in an atmosphere comprising air, an oxidizing gas, nitrogen, an inert gas, or combinations thereof. Typical heat-treating times are dependent on the heat-treating temperature. Suitable heat-treating conditions for a carbon nanotube film can comprise heating at 200° C. for 2 to 3 hours in nitrogen or air. High temperature heat-treatment conditions can comprise 900° C. in nitrogen for about 2 minutes.

Heat-treatment temperatures in nitrogen or inert environments can typically be higher than in oxidative environments. Typically, heat-treatment in nitrogen or inert gases can be done at temperatures between about 200° C. to about 1000° C. Heat-treatment in air or oxidative atmospheres can be done at temperatures up to about 1000° C. for some mild oxidizing agents such as carbon dioxide. In one embodiment, the duration of the heat-treating is about 2 hours at 200° C. in air, nitrogen or an inert atmosphere. The thickness of the carbon nanotube film can range from about 0.1 micron to about 10,000 microns. Typically, the thickness of the carbon nanotube film can range from about 1 micron to about 1,000 microns, and, even more typically, the thickness of the nanotube film can range from about 1 micron and about 100 microns.

The resulting heat-treated film consists essentially of small-diameter single-wall carbon nanotubes and/or small-diameter multi-wall carbon nanotubes that have been oxidized. The heat-treated carbon nanotube film can further comprise crosslinked carbon nanotubes. Typically, the film comprises primarily small-diameter carbon nanotubes or primarily small-diameter single-wall and/or small-diameter multi-wall carbon nanotubes.

In another embodiment, the heat-treated carbon nanotube film comprises primarily oxidized small-diameter carbon nanotubes, e.g. primarily oxidized small-diameter single-wall and/or small-diameter multi-wall carbon nanotubes, and has a tensile strength of at least about 70 MPa.

In another embodiment, the heat-treated small-diameter carbon nanotube film comprises at least about 50 wt % small-diameter carbon nanotubes and has a tensile strength of at least about 70 MPa and high initial tensile modulus of about 5 GPa. In another embodiment, the carbon nanotubes are purified.

In another embodiment, the heat-treated small-diameter carbon nanotube film comprises primarily oxidized small-diameter carbon nanotubes, e.g. primarily oxidized small-diameter single-wall carbon nanotubes and/or oxidized small-diameter multi-wall carbon nanotubes, and has a tensile strength of at least about 15 MPa, at least about 25 MPa, or at least about 50 MPa.

In another embodiment, a film comprises at least about 80 wt % oxidized single-wall carbon nanotubes and/or oxidized small-diameter multi-wall carbon nanotubes, wherein the film has a tensile strength of at least about 15 MPa, at least about 25 MPa, at least about 50 MPa, or at least about 70 MPa.

In one embodiment of the present invention, a conductive small-diameter carbon nanotube film is prepared with small-diameter carbon nanotubes that have been oxidatively treated with nitric acid, and has a substantially higher tensile strength and initial tensile modulus than a film prepared with small-diameter carbon nanotubes without nitric acid treatment. For example, in one embodiment of the present invention, a carbon nanotube film comprising small-diameter carbon nanotubes is prepared from small-diameter carbon nanotubes treated in 10M nitric acid and has a tensile strength and an initial tensile modulus of about 74 MPa and about 5 GPa, respectively. These strength and modulus values are about seven and six times greater, respectively, than those for comparable film made with nanotubes without nitric acid treatment.

In another embodiment, a film comprising small-diameter carbon nanotubes that have been treated with nitric acid, has an electrical conductivity value on the order of about 10⁴ Siemens/m, which is comparable to the electrical conductivity of films prepared with nanotubes without nitric acid treatment and also comparable to electrically conductive polymers, such as polythiophene and polypyrrole.

In another embodiment, a film comprising oxidized small-diameter carbon nanotubes has a stable mechanical performance over a large temperature range. The stable mechanical performance in an oxidative environment is expected to be up to about 400° C., and up to about 1000° C. in nitrogen or an inert environment.

The following examples are included to demonstrate preferred embodiments of the invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples which follow represent techniques discovered by the inventor to function well in the practice of the invention, and thus can be considered to constitute preferred modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention.

EXAMPLE 1

This example demonstrates a method for preparing a high strength film comprising predominantly single-wall carbon nanotubes (SWNT). Purified, HIPCO® small-diameter carbon nanotubes, comprising almost entirely single-wall carbon nanotubes, obtained from Rice University and Carbon Nanotechnologies, Inc., were made in a high temperature, high pressure, gas phase process through the disproportionation of carbon monoxide to primarily single-wall carbon nanotubes and CO₂ using iron as the transition metal catalyst. (HIPCO is a registered trademark of Carbon Nanotechnologies, Incorporated, Houston, Tex.) 100 mg purified small-diameter carbon nanotubes were dispersed in 100 mls distilled water and sonicated for 2 hours using a Fisher Scientific bath sonicator (frequency 43 KHz, power 150 Watts). Nitric acid was then added to the dispersion to obtain a nitric acid concentration of 3M, 6M, or 10M. Each dispersion was then sonicated for 2 more hours, then refluxed for two hours, and subsequently sonicated for another 20 minutes. Each dispersion was filtered through a polytetrafluoroethylene (PTFE) membrane filter (Gelman Laboratory, 1-μm pore diameter) and repeatedly washed with distilled water. The resulting carbon nanotube films comprising small-diameter nanotubes were easily peeled off the PTFE membrane filter.

A control sample was made by sonicating an aqueous dispersion of small-diameter carbon nanotubes without any nitric acid for 4 hours, filtering the dispersion through a PTFE membrane filter and peeling off a small-diameter carbon nanotube film from the PTFE membrane.

Each film was dried at 70° C. in a vacuum, and heat-treated at 200° C. for 2 hrs in air. The heat-treated films made in this example are referred to the “as prepared” film samples in further examples.

EXAMPLE 2

Tensile mechanical properties and dc conductivity were measured on the small-diameter carbon nanotube films prepared in Example 1. The film samples tested in the tensile tests were 1 mm wide and 0.06 mm thick. The gauge length was 10 mm between the clamps. The tensile tests were conducted on a Rheometrics Solids Analyzer, RSA III, at a strain rate of 0.5% per second. The tensile test data are summarized in Table 1.

The mechanical studies showed a tensile strength of 74 MPa for the “as prepared” (heat-treated at 200° C.) small-diameter carbon nanotube film prepared with 10M nitric acid, a greater than seven-fold increase in tensile strength over the control film, which had a tensile strength of 10 MPa. The results also showed an initial tensile modulus of 5.0 GPa for the heat-treated small-diameter carbon nanotube film prepared with 10M nitric acid, a greater than six-fold increase in initial tensile modulus over the control film, which had an initial tensile modulus of 0.8 MPa. Mechanical tests on films made with small-diameter carbon nanotubes processed with 6M and 10M nitric acid also showed that the 200° C. heat-treatment increased the tensile strength and tensile modulus over the films that were only dried. The heat-treatment did not substantially change the elongation-to-break or the electrical conductivity.

In-plane dc electrical conductivity of the films was measured at room temperature by the four-probe method. Conductivity is quantified in units of Siemens (S) per unit length, such as S/cm or S/m. (Note: Siemen=mho=1/ohm.) Resistivity, the inverse of conductivity, is quantified in units of ohm-length, such as ohm-cm.

The electrical conductivity results for the small-diameter carbon nanotube films are also given in Table 1 below. The results show that the in-plane dc electrical conductivity for the “as prepared” small-diameter carbon nanotube films made with nitric acid treatment is on the same order of magnitude as the control film made without nitric acid treatment. The “as prepared” film made with nanotubes treated in 10 M nitric acid had a conductivity of 1.2×10⁴ Siemens/m versus 3×10⁴ Siemens/m for the control film. Although not meant to be held by theory, the somewhat lower conductivity for the carbon nanotube films made with nitric acid-treated nanotubes could be attributed to acid-induced creation of defects in the small-diameter carbon nanotube structure and the presence of functional groups at the defect sites. The presence of functional groups on the acid-treated carbon nanotubes provides for enhanced inter-tube interaction, as well as for the possibility of crosslinking. The somewhat lower conductivity could be also due to reduced catalytic impurity, as well as a possible reduction in metallic nanotube population. TABLE 1 Mechanical and electrical properties of various carbon nanotube films Initial Strain-to- Tensile Tensile Failure Small-Diameter Carbon Strength Modulus Elongation Conductiv- Nanotube Films (MPa) (GPa) (%) ity (S/m) Control 10 ± 2 0.8 ± 0.1 5.6 ± 0.3 3.0 × 10⁴ Prepared in 3 M HNO₃ 16 ± 1 1.4 ± 0.1 1.4 ± 0.2 2.3 × 10⁴ Prepared Before 200° 56 1.5 4.8 3.1 × 10⁴ in 6 M C. heat treat- HNO₃ ment After 200° 71 ± 5 2.9 ± 0.2 3.4 ± 0.2 2.4 × 10⁴ C. heat treat- ment Prepared Before 200° 68 4.5 3.0 1.3 × 10⁴ in 10 M C. heat treat- HNO₃ ment After 200° 74 ± 2 5.0 ± 0.2 3.0 ± 0.1 1.2 × 10⁴ C. heat treat- ment

EXAMPLE 3

Dynamic mechanical analysis (DMA) as a function of temperature was done on the “as prepared” (200° C. heat-treated) small-diameter carbon nanotube films, prepared in Example 1, at a frequency of 10 Hz and at 0.1% dynamic strain using a RSA III Rheometrics Solids Analyzer. During the dynamic test, the static force adjusted automatically to 40% larger than dynamic force.

The results show a fairly constant E′ storage modulus for the “as prepared” small-diameter carbon nanotube films throughout the temperature range of about 30° C. to about 210° C., with a slight increase in storage modulus above 150° C. The Tan δ values were very low (i.e., about 0.02), indicating that the films are fairly elastic throughout the 30° C. to about 210° C. temperature range. A plot of the dynamic mechanical behavior for the “as prepared” film made with small-diameter carbon nanotubes processed in 6 M nitric acid is shown in FIG. 1.

EXAMPLE 4

The effects of heat treating on the films of small-diameter carbon nanotubes were studied by Scanning Electron Microscopy (SEM). The “as prepared” films made with small-diameter carbon nanotubes treated in nitric acid were compared to the same films after an additional high temperature heat treatment. “High temperature heat-treated” films were prepared from the “as-prepared” films by heating the “as-prepared” films in nitrogen in a TGA to 900° C. at a rate of 20° C./minute and then holding for 2 minutes at 900° C.

Scanning electron micrographs of the “as-prepared” heat-treated films are shown in FIGS. 2A, 2B and 2C, which correspond to films prepared from small-diameter carbon nanotubes treated in 3M, 6M and 10M nitric acid, respectively. Scanning electron micrographs of the “high temperature heat treated” films are shown in FIGS. 2D, 2E and 2F, made by heat-treating the “as prepared” films shown in FIGS. 2A, 2B and 2C, prepared with small-diameter carbon nanotubes processed in 3M, 6M and 10M nitric acid, respectively, at 900° C. as described above.

FIGS. 2A and 2D are SEMs showing nanotube ropes on the surface of the “as prepared” heat-treated and “high temperature heat-treated” films, respectively, in which the nanotubes were processed in 3M nitric acid. Ropes are not seen on the film surfaces of the “as prepared” heat-treated films made with nanotubes processed in 6 M nitric acid (See FIG. 2B) and 10 M nitric acid (FIG. 2C). However, after the “as prepared” films were heat-treated at 900° C., the nanotube ropes can be clearly seen on both the surfaces of the “high temperature heat-treated” films. FIG. 2E shows a SEM of the “high temperature heat-treated” film made from the “as prepared” film, shown in FIG. 2B, which was made from nanotubes processed in 6M nitric acid. FIG. 2F shows a SEM of the “high temperature heat-treated” film made from the “as prepared” film, shown in FIG. 2C, which was made from nanotubes processed in 10M nitric acid. Although not meant to be held by theory, the material surrounding the nanotube ropes in films made with 6M and 10M-nitric acid treated nanotubes is attributed, at least in part, to amorphous carbon resulting from acid treatment-induced nanotube decomposition. The 900° C. heat-treatment of the films made with 6M nitric acid- and 10M nitric acid-treated nanotubes appears to have removed the continuous phase attributed to amorphous carbon from the surface of the films so that the nanotube bundles can be clearly observed. These observations support the theory that the films prepared from nitric acid-treated small-diameter carbon nanotubes have a composite structure comprising nanotube ropes embedded in a continuous matrix phase attributed to amorphous carbon and polycyclic aromatic material, which was expected to have formed, at least in part, from the decomposition of small-diameter carbon nanotubes. The appearance of an amorphous phase is consistent with the disappearance of small diameter nanotubes as the concentration of nitric acid treatment was increased.

EXAMPLE 5

Raman studies were done on the “as prepared” and “high temperature heat-treated” films comprising with small-diameter carbon nanotubes processed in various concentrations of nitric acid, as prepared in Example 1. “High temperature heat-treated” films were prepared by heating the “as-prepared” films in nitrogen in a thermogravimetric analyzer (TGA) at rate of 20° C./minute to 900° C. and holding at 900° C. for two minutes.

Raman spectra were collected using a Holoprobe Research 785 Raman Microscope made by Kaiser Optical System, Inc. with an incident laser wavelength of 785 nm. Diameter determinations were made using the Raman radial breathing modes (RBM) peaks in the range of about of 150 to 350 cm⁻¹ that correlate with carbon nanotube diameters. The nanotube diameters were calculated based on the RBM band peak position of the control film using the empirical equation ω_(RBM)=238/d^(0.93). Metallic carbon nanotube configurations cannot be observed using a 785-nm laser wavelength, thus the present Raman observations and analyses are limited to semiconducting tubes that are observable with the 785-nm incident wavelength.

No RBM peaks in the Raman spectra were observed for “as produced” films.

Raman RBM peak spectra for the 900° C. “high temperature heat-treated” films are shown in FIGS. 3A-D. FIG. 3A shows a dominant RBM peak at about 267 cm⁻¹ for the control film, while the dominant RBM peak shifts to lower frequency for samples made with increasing nitric acid concentrations, consistent with the destruction of smaller diameter tubes and survival of larger diameter tubes. The stress-induced curvature around the circumference of the nanotubes makes small-diameter nanotubes more reactive and liable to chemical attack than larger diameter nanotubes.

The changes in the diameter distribution of the nanotubes resulting from the oxidative nitric acid treatment were quantified by resolving the spectra in FIGS. 3A-D, using a Lorentz line shape fitting routine. Using a 785-nm incident laser wavelength, six different diameters in the range of 0.88-1.19 nm were identified in these films. The RBM peaks for carbon nanotube films made with nitric acid-treated nanotubes were also up-shifted by 1-3 cm⁻¹ compared to the corresponding peaks for the control film processed without nitric acid.

The relative fraction of a particular diameter of nanotube was determined by taking the ratio of the area of the corresponding peak to the sum of the area of all the RBM peaks. FIGS. 4A, 4B, 4C, 4D, 4E and 4F show plots of the relative fractions of small-diameter carbon nanotubes having diameters of 1.19 nm, 1.11 nm, 1.07 nm, 1.02 nm, 0.89 nm and 0.88 nm, respectively, in “high temperature heat-treated” small-diameter carbon nanotubes films as a function of nitric acid concentration. FIGS. 4E and 4F show that the relative fractions of 0.88-nm and 0.89-nm small-diameter nanotubes, respectively, significantly decreased with increasing nitric acid concentration. That the percentage of these small-diameter nanotubes decreased from 70% in the control film to less than 20% in samples processed in 10 M HNO₃, supports the theory of selective degradation of the small-diameter carbon nanotubes by nitric acid. The destruction of small-diameter nanotubes, corresponds to an increase of the relative fraction of the 1.19-nm and 1.11-nm diameter small-diameter carbon nanotubes, the amount of which increased, generally monotonically, with increasing nitric acid concentration.

The chemical reactivity of small-diameter carbon nanotubes with respect to their diameter is further demonstrated by the relative population changes of 1.02 and 1.07-nm diameter nanotubes. The relative fraction of 1.07-nm diameter nanotubes was fairly constant when processed in 6 M and 10 M HNO₃, however, the 1.02-nm diameter fraction decreased when processed in 6 M nitric acid and further decreased when processed in 10 M nitric acid. This suggests that the 1.07-nm diameter nanotubes have higher resistance to oxidative degradation than the 1.02-nm diameter nanotubes. Although the diameter difference between these two tubes is only 5%, there is a measurable difference between their oxidative resistances. In FIGS. 4A-F, the circle symbol in the plots represents the relative fraction of small-diameter carbon nanotubes in a film sample where the nanotubes were processed in 6 M nitric acid and the film was subjected to a more severe heat-treatment of 900° C. for 30 minutes and 700° C. for 4.5 hrs in N₂. The similarity in the relative populations between the high temperature treatment (squares) and the more severe heat treatment (circles) suggests that little or no additional nanotube degradation occurred by subjecting the nanotubes to the more severe heat treatment. These results are consistent with the higher propensity for the smaller diameter carbon nanotubes to be attacked and damaged first by the nitric acid.

EXAMPLE 6

The “as-prepared” films made with small-diameter carbon nanotubes processed in 3M, 6M and 10M nitric acid as described in Example 1 were analyzed by thermogravimetric analysis (TGA) using a TA Instruments TGA2950. The samples were heated to 900° C. at 10° C./minute in nitrogen. Ash residues were 80, 65, 57, and 52 wt % for the small-diameter nanotube control, and the nanotubes processed in 3M, 6M, and 10M nitric acid, respectively. The results are consistent with the production of more amorphous carbon with increasing nitric acid concentration.

All of the compositions and methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the invention. More specifically, it will be apparent that certain agents which are chemically related may be substituted for the agents described herein while the same or similar results would be achieved. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined by the appended claims. 

1. A method for making a conductive carbon nanotube film, comprising: (a) refluxing an aqueous mixture comprising carbon nanotubes and an oxidizing agent to form a refluxed nanotube dispersion; (b) forming a carbon nanotube film from the refluxed nanotube dispersion; (c) drying the carbon nanotube film; and (d) heat-treating the carbon nanotube film to form a heat-treated carbon nanotube film.
 2. The method of claim 1, wherein the carbon nanotubes are purified.
 3. The method of claim 1, wherein the carbon nanotubes comprise single-wall carbon nanotubes.
 4. The method of claim 1, wherein the carbon nanotubes comprise multi-wall carbon nanotubes, wherein the multi-wall carbon nanotubes have diameters at most about 3 nm.
 5. The method of claim 1, wherein the oxidizing agent is nitric acid.
 6. The method of claim 5, wherein the concentration of nitric acid in the aqueous mixture is in the range of about 3 Molar and about 10 Molar.
 7. The method of claim 5, wherein the concentration of nitric acid in the aqueous mixture is in the range of about 3 Molar and about 6 Molar.
 8. The method of claim 5, wherein the concentration of nitric acid in the aqueous mixture is in the range of about 6 Molar and about 10 Molar.
 9. The method of claim 1, wherein the oxidizing agent is selected from the group consisting of ozone, potassium persulfate, a mixture of nitric acid and sulfuric acid, a mixture of nitric acid and hydrogen peroxide, steam, carbon dioxide, halogens, halogen-containing compounds, and combinations thereof.
 10. The method of claim 1, further comprising removing the oxidizing agent from the carbon nanotube film.
 11. The method of claim 10, wherein the removing is done by washing with a solvent selected from the group consisting of acetone, alcohol, water and a combination thereof.
 12. The method of claim 1, wherein the forming is done by filtering the carbon nanotubes.
 13. The method of claim 1, wherein the forming is done on an adsorbent or non-adsorbent surface.
 14. The method of claim 1, wherein the drying is done in a vacuum.
 15. The method of claim 1, wherein the drying is done in an atmosphere selected from the group consisting of a vacuum, nitrogen and inert gas.
 16. The method of claim 1, wherein the drying is done at a temperature in the range of about 15° C. and about 200° C.
 17. The method of claim 1, wherein the heat-treating is done in an oxygen-containing atmosphere.
 18. The method of claim 1, wherein the heat-treating is done in an inert atmosphere.
 19. The method of claim 1, wherein the heat-treating is done at a temperature in the range of at least about 200° C. and about 1000° C.
 20. The method of claim 1, wherein the heat-treated carbon nanotube film has a tensile strength of at least about 15 MPa.
 21. The method of claim 1, wherein the heat-treated carbon nanotube film has a tensile strength of at least about 25 MPa.
 22. The method of claim 1, wherein the heat-treated carbon nanotube film has a tensile strength of at least about 50 MPa.
 23. The method of claim 1, wherein the heat-treated carbon nanotube film has a tensile strength of at least about 70 MPa.
 24. The method of claim 1, wherein the carbon nanotube film comprises crosslinked carbon nanotubes.
 25. The method of claim 1, further comprising sonicating the carbon nanotubes in water before refluxing.
 26. The method of claim 1, further comprising sonicating the mixture, the dispersion or both.
 27. The method of claim 1, wherein the heat-treated carbon nanotube film comprises primarily single-wall carbon nanotubes.
 28. The method of claim 1, wherein the heat-treated carbon nanotube film comprises primarily small-diameter carbon nanotubes having diameters at most about 3 nm.
 29. The method of claim 1, wherein the heat-treated carbon nanotube film has a thickness in the range of about 0.1 micron and about 10,000 microns.
 30. The method of claim 1, wherein the heat-treated carbon nanotube film has a thickness in the range of about 1 micron and about 1,000 microns.
 31. The method of claim 1, wherein the heat-treated carbon nanotube film has a thickness in the range of about 1 micron and about 100 microns.
 32. An film comprising primarily small-diameter carbon nanotubes, wherein the nanotubes have been oxidized, and wherein the film has a thickness in the range of about 0.1 micron and about 10,000 microns and tensile strength of at least about 15 MPa.
 33. The film of claim 32, wherein the film has a tensile strength of at least about 25 MPa.
 34. The film of claim 32, wherein the film has a tensile strength of at least about 50 MPa.
 35. The film of claim 32, wherein the film has a tensile strength of at least about 70 MPa.
 36. The film of claim 32, wherein the film has a thickness in the range of about 1 micron and about 1,000 microns.
 37. The film of claim 32, wherein the film has a thickness in the range of about 1 micron and about 100 microns.
 38. A film consisting essentially of small-diameter carbon nanotubes, wherein the small-diameter carbon nanotubes are crosslinked.
 39. The film of claim 38, wherein the film has a tensile strength of at least about 25 MPa.
 40. The film of claim 38, wherein the film has a tensile strength of at least about 50 MPa.
 41. The film of claim 38, wherein the film has a tensile strength of at least about 70 MPa.
 42. The film of claim 38, wherein the film has a thickness in the range of about 0.1 micron and about 10,000 microns.
 43. The film of claim 38, wherein the film has a thickness in the range of about 1 micron and about 1,000 microns.
 44. The film of claim 38, wherein the film has a thickness in the range of about 1 micron and about 100 microns.
 45. A conductive carbon nanotube film made by the process comprising: (a) refluxing an aqueous mixture comprising carbon nanotubes and an oxidizing agent to form a refluxed nanotube dispersion; (b) forming a carbon nanotube film, (c) drying the carbon nanotube film; and (d) heat-treating the carbon nanotube film to form a heat-treated carbon nanotube film.
 46. The film of claim 45, wherein the carbon nanotubes are purified.
 47. The film of claim 45, wherein the carbon nanotubes comprise single-wall carbon nanotubes.
 48. The film of claim 45, wherein the carbon nanotubes comprise small-diameter carbon nanotubes, wherein the small-diameter carbon nanotubes have diameters of at most about 3 nm.
 49. The film of claim 45, wherein the oxidizing agent is nitric acid.
 50. The film of claim 45, wherein the oxidizing agent is removed from the carbon nanotubes.
 51. The film of claim 45, wherein the forming is done by filtering.
 52. The film of claim 45, wherein the heat-treating is done at a temperature in the range of at least about 200° C. and about 1000° C.
 53. The film of claim 45, wherein the heat-treated carbon nanotube film has a thickness in the range of about 0.1 micron and about 10,000 microns.
 54. The film of claim 45, wherein the heat-treated carbon nanotube film has a thickness in the range of about 1 micron and about 1,000 microns
 55. The film of claim 45, wherein the heat-treated carbon nanotube film has a thickness in the range of about 1 micron and about 100 microns. 